This invention relates generally to apparatus for polishing spherical objects, and more particularly the invention relates to a novel low stress apparatus for and method of polishing of such objects.
Spherical objects such as yig crystals and steel balls for ball bearings, for example, require a uniform polished surface. Yig crystals are used in microwave electronic circuits as tuning elements. Typically, a thin wafer of a yig crystal is scribed and broken to obtain a plurality of cubical dice. Corners of the dice are broken away in a tumbler, and then the dice are polished in a lapping machine to obtain a spherical configuration.
Heretofore, the dice have been placed between two plates having sandpaper surfaces, and then the plates are rotated in pressure engagement to lap the dice crystals. Actual lapping time is on the order of 38 hours, however, due to stop-start time associated with monitoring the polishing process and maintaining the polishing machine during operation, actual time for polishing a batch of yig crystals averages 23 days. Since an operator must be present during the lapping operation, a single working shift is limited to eight hours per day, with actual polishing time being on the order of five hours. The process must be periodically stopped and spheres removed from the machine to measure diameter and to remove broken or damaged spheres. Additionally, the sandpaper must be periodically changed.
The conventional polishing apparatus relies on several grades of sandpaper to accomplish the rough and fine grinding of spheres. Typically, course and fine polish are done with a unique non-woven dense polishing cloth and alumina abrasives. Brass backing plates are used to support surfaces of paper on the moving and stationary lapping surfaces, and a composite fiber material is used to form a retaining ring to prevent the spheres from ejecting from the machine due to angular acceleration.
Additionally, conventional polishing machines exert several pounds of pressure on the particles between the plates which results in a relatively high rate of stock removal. The amount of material being removed must be constant and without preference to any sector of the surface, excessive down force per unit area inhibits the sphere from rotating on a random axis as is required for spherical polishing.
Another cause of non-uniform stock removal is skidding rather than rolling, of particles between the lapping plates. This is caused by non-flatness of the lapping surfaces, and damage is exacerbated when coupled with high rotational speeds. Skidding is particularly damaging, and may have permanent effects. At the beginning of the process when particles exhibit only an approximately round shape and are prone to skidding anyway. Particles that are highly spherical are less susceptible to skidding and may tolerate higher rotational speeds without sustaining damage. The non-flatness allows spheres in process to decelerate in rotational speed as they disengage from both top and bottom surfaces. Damage occurs when the spheres re-engage the top and bottom lapping surfaces and experience very high torque loads and rates of acceleration. In effect, a sphere is not able to instantaneously accelerate to the angular surface speed of the lapping wheel and consequently skids on the lapping surface.